Since ceramic-based composite materials obtainable by reinforcing ceramics with inorganic fibers have high toughness and high strength and are excellent in heat resistance, developments thereof are vigorously made. A SiC/SiC composite material obtainable by reinforcing a silicon carbide matrix with a silicon carbide fiber is one of the most promising materials in a high-temperature use.
It is widely known that characteristics of an interface between the fiber and the matrix are very important for controlling the mechanical properties of the SiC/SiC composite material. That is, a difference in the interfacial bonding strength of the fiber and the matrix exerts a great influence on the mechanical strength of the composite material. When the interfacial bonding of the fiber and the matrix is too large, cracks generated in the matrix easily spread into the fiber so that the material shows very brittle breakdown behavior and its strength and toughness become very low. On the other hand, when the interfacial bonding of the fiber and the matrix is appropriate, bridging or pull-out of the fiber occurs at the destruction of the material so that favorable strength and toughness are shown. For controlling the interfacial bonding strength of the fiber and the matrix, generally, an interface layer is formed on a fiber surface.
As an interface material, there are used carbon, boron nitride and the like. Of these interface materials, boron nitride is the most excellent in heat resistance and receives attention.
When boron nitride is used as an interface material for a ceramic-based composite material, it is required to form a boron nitride layer on a reinforcing fiber surface in some way.
Generally, for example, as shown in Ceramic Engineering Science Proceedings 16 (4) (1995), p 405–416, the formation of boron nitride layer is carried out by using boron trichloride or boron trifluoride and ammonia as a raw material gas according to a chemical vapor deposition method (CVD method) However, the CVD method requires a special CVD device and the raw material gas is expensive and dangerous, so that it is a very high-cost process.
Further, for example, as shown in Journal of American Ceramic Society vol. 77 No. 4, p 1,011–1,016, there has been used a method in which a fiber is immersed in a boric acid solution and then the fiber is sintered in an ammonia atmosphere to form a boron nitride layer on a fiber surface. However, in the solution immersion method like above, it is very difficult to form a boron nitride layer having a uniform thickness on the surface of each fiber of a fiber bundle. In most cases, there is caused bridging in which fibers are bonded to each other with boron nitride layers.
Further, M. D. Sacks et al. disclose a method for forming a boron nitride layer on a silicon carbide fiber surface in U.S. Pat. No. 6,040,008 and Ceramic Engineering and Science Proceedings Volume 21, Issue 4(2,000), p 275–281. In this method, a boron compound is added to a high molecular-weight polycarbosilane having a weight average molecular weight of 7,000 to 16,000, the mixture is dry-spun to form a spun fiber, the spun fiber is sintered in argon gas to introduce boron into a silicone carbide fiber homogeneously, and the resultant fiber is again sintered in a nitrogen-containing atmosphere, thereby forming a boron nitride layer on a silicon carbide fiber surface. However, in the case of the above method, it is very difficult to make the boron homogeneously dispersed in the silicon carbide fiber move to the fiber surface by heat treatment. Therefore, for forming a boron nitride layer having a sufficient thickness as an interface layer on the fiber surface, it is indispensable to increase the amount of boron to be introduced into the silicon carbide fiber. However, the oxidation resistance of the fiber becomes worse as the boron amount in the silicon carbide fiber increases. Further, according to the above document, the silicon carbide layer formed by the above method is arranged in a direction perpendicular to a fiber axis direction so that the boron nitride layer having such a structure can not be expected to exert a big effect as an interface layer for a ceramic-based composite material.